1. Field of the Invention
The subject of the present invention is a device for improving the contrast of a liquid crystal screen by using an opaque mask (commonly called "Black Matrix") and its method of manufacture.
2. Discussion of the Background
A liquid crystal screen consists of a number of liquid crystal cells disposed in a matrix lattice. These cells are formed from two support plates, in general made of glass, stuck together by a peripheral seal which makes a cavity with a small thickness (of the order of a few microns) in which the optically active element, the liquid crystal, is situated. The first support plate consists of a substrate containing a first set of electrodes and the thin film control components disposed between these electrodes to which they are connected. The second support plate constitutes a back-plate electrode.
Each cell thus formed functions as an optical valve. The local modification of the transmission or of the reflection of light is obtained by externally applying a voltage between an access contact of the plate and a contact of the back plate. This voltage gives rise to an electric field between the opposite electrodes and activates the volume of liquid crystal situated between the two electrodes which modifies to a greater or lesser extent the characteristics of the light which passes through it.
In the rest of the text, the term Pixel (short for Picture Element in English) is used to mean the cell forming the elementary image or display point constituted by the electrode, the liquid crystal, the back-plate electrode, the control component and the connections of the component with the peripheral electronics.
The active surface of the Pixel is the surface covered by the electrode and the liquid crystal on which the desired electrooptical effects are obtained; this surface may be larger or smaller in area than the effective surface of the electrode because the edge effects around the electrode may either increase or diminish the surface area of the active zone (these variations being of the order of 1 .mu.m short of or beyond the zone covered by the electrode).
The noncontrolled surface of the Pixel is constituted by the whole surface of the Pixel outside the active surface previously defined.
The use of an opaque mask (or Black Matrix), for improving the contrast, in order to hide the interstices of a liquid crystal or cathode ray tube display is known. In the case of a liquid crystal screen it is a mask covering the noncontrolled surfaces of the Pixels. This opaque mask, which may be made of a reflecting material, blocks the transmission of the light over the uncontrolled matrix surface and allows the contrast of the screen to be improved.
In the prior art, this opaque matrix mask is drawn on the glass back plate constituting the back-plate electrode of the cell. During this method of manufacture, this glass back plate is positioned with respect to the plate forming the substrate on which the electrodes are deposited and stuck to this plate by a peripheral seal. The precision of this positioning is in general limited to within 5 .mu.m. In order to integrate this uncertainty due to the method of sticking used, the opaque mask which is superimposed over the pixels must be oversized with respect to the dimensions of the noncontrolled surface and will obscure a not insignificant part of the active surface of these pixels. For medium or good resolution screens, that is to say screens whose lattice pitch is of the order of 150 to 250 .mu.m, as in the case of direct vision screens, transmission of the light is not significantly reduced. For so-called high definition screens, by contrast, as for projector screens, this loss of active surface causes a significant loss of transmission of the light. For example for a screen with a 50 .mu.m pitch, the active surface of the pixels will measure approximately 40 .mu.m on each side, i.e. 64% of the total surface area of the pixel. If the opening in the opaque mask extends 10 .mu.m beyond the active surface of the pixels, this now represents only 36% of the total surface area of the pixel.